1. Field of the Invention
The invention relates to a zoom lens having an ultra wide angle of view and large aperture, and an imaging optical device including the zoom lens.
2. Description of the Related Art
Video cameras or digital cameras including a solid-state imaging device, such as a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS), have been widely used in recent years. In particular, the demand for megapixel camera modules is increasing, and high quality digital cameras with over 5 million pixels are now available. Digital cameras using an imaging device, such as a CCD or a CMOS, or imaging optical devices, such as mobile phone cameras, are required to be compact, light, and inexpensive, and are also required to have a wide angle of view in order to secure a sufficient visual field in a wide area.
For example, digital cameras, or optical systems using an interchangeable lens or a solid-state imaging device are required to have high resolution, a wide angle of view, and high magnification, and are also required to have a low F-number (FNO) in order to achieve high brightness. Some zoom lenses have a wide angle of view or high magnification, and other zoom lenses have a low FNO and high brightness and include an optical system with a fixed focal length. However, zoom lenses having a wide angle of view, high magnification, and a low FNO have not been developed yet due to technical difficulties.
While many 3-group zoom lenses have been developed to reduce the size of an optical system, it is still difficult for 3-group zoom lenses to have a wide angle of view and high magnification. Also, if zoom lenses having a low FNO are 3-group zoom lenses, a change in the FNO during zooming is great, and thus brightness often is high at a wide-angle position and is low at a telephoto position. It is also difficult for 4-group zoom lenses to have a wide angle of view, high magnification, high brightness, and compact design.